In recent years, there has been active research conducted on organic-inorganic composite materials, in which inorganic nanoparticles are dispersed in thermoplastic resins to improve physical properties of the thermoplastic resins, such as mechanical strength, heat resistance, coefficient of linear thermal expansion, and inflammability. Because the inorganic nanoparticles are inorganic materials, the inorganic nanoparticles have poor compatibility with the thermoplastic resins, which are organic materials, and are difficult to disperse uniformly in the thermoplastic resins. Also, the inorganic nanoparticles, due to their nano-sized dimensions, have very large cohesion force working between the particles compared to that of the micron-sized particles. Accordingly, it is difficult to disperse the inorganic nanoparticles uniformly in the thermoplastic resins because of their material and dimensions.
As a material which contains inorganic nanoparticles uniformly dispersed in a thermoplastic resin, there are generally known a material obtained by using a dispersion stabilizer agent, a material obtained by modifying the thermoplastic resin, and a material obtained by modifying the surface of the inorganic nanoparticles. As the material obtained by using a dispersion stabilizer agent, a clay composite material has been proposed (see, for example, Patent Document 1), in which a lamellar clay mineral functioning as the inorganic nanoparticles is dispersed in a polyolefin resin by using, as the dispersion stabilizer agent, a polyolefin polymer having a functional group capable of hydrogen-bonding with the lamellar clay mineral. By using the clay composite material, mechanical properties such as contraction percentage can be improved.
As the material obtained by modifying a thermoplastic resin, there has been proposed a light-transmissive resin composition (see, for example, Patent Document 2), wherein acidic groups capable of forming hydrogen-bond with inorganic fine particles are introduced in the light-transmissive thermoplastic resin when dispersing the inorganic nanoparticles in the light-transmissive resin. The light-transmissive resin composition can decrease orientation birefringence while maintaining transparency.
As the material obtained by modifying the surface of the inorganic nanoparticles, there has been proposed a nano-composite transparent resin composition in which a silica composition is dispersed in a polymer material having phenyl groups within the molecule and the surface layer of the silica composition is modified with the phenyl groups when dispersing (see, for example, Patent Document 3). This type of nano-composite transparent resin composition has excellent transparency and can improve rigidity.
In addition, the following graft copolymer-containing metal oxide fine particles is known (see, for example, Patent Document 4). Namely, this is a material in which a vinyl monomer is graft-polymerized to a composite rubber which comprises the metal oxide fine particles, polyorganosiloxane, and polyalkyl (meth)acrylate. The graft copolymer-containing the metal oxide fine particles has excellent impact strength, surface hardness, and the like, and is used alone or as a mixture with a thermoplastic resin.
Furthermore, as the material obtained by modifying the surface of the inorganic nanoparticles, there has been proposed a polymer composite material (see, for example, Patent Document 4), which is organized with polyfunctional amines capable of forming ionic bonds with a lamellar clay mineral when dispersing the lamellar clay mineral functioning as inorganic nanoparticles in a polyamide resin and, thereafter, is dispersed in the polyamide resin to form hydrogen bonds with the polyamide resin. This type of polymer composite material has excellent mechanical strength and toughness.    Patent Document 1: Japanese Patent No. 3489411 (pages 1 and 2)    Patent Document 2: Japanese Patent Laid-Open No. 2004-217714 (pages 2 and 3)    Patent Document 3: Japanese Patent Laid-Open No. 2004-2605 (pages 2 and 5)    Patent Document 4: Japanese Patent No. 2519045 (pages 1 and 2)    Patent Document 5: Japanese Patent Laid-Open No. H8-325340 (pages 2 and 4)